The Fas protein is a type I membrane protein that belongs to the tumor necrosis factor (TNF) receptor family (see S. Nagata et al., Science, 267:1449, 1995). Many tissues and cell lines weakly express Fas, but abundant expression is found in the heart, lung, liver, ovary and thymus (R. Watanabe-Fukunaga et al., J. Immunol. 148:1274, 1992). In addition, Fas is highly expressed on activated lymphocytes including T cells (Nagata et al., op. cit.). Fas transmits a signal for apoptosis or programmed cell death (see C. Thompson, Science 267:1456, 1995) when it is triggered by binding of certain antibodies such as APO-1 (B. Trauth et al., Science 245:301, 1989) and anti-Fas (S. Yonehara et al., J. Exp. Med. 169:1747, 1989). Apoptotic cell death is characterized by nuclear and cytoplasmic shrinkage, membrane blebbing, and degradation of chromosomal DNA in a characteristic pattern, and can be distinguished from necrotic cell death due to acute cellular injury (Thomson, op. cit.).
The natural ligand for Fas is known simply as the Fas ligand (FasL). Its rat (T. Suda et al., Cell 75:1169, 1993), mouse (D. Lynch et al., Immunity 1:131, 1994; T. Takahashi et al., Cell 76:969, 1994) and human (T. Takahashi et al., Internat. Immunol. 6:1567, 1994) forms have been cloned at the cDNA level. FasL is a type II membrane protein, i.e, having an extracellular carboxyl terminal domain and an intracellular amino terminal domain, and belongs to the TNF family of proteins (T. Suda et al., op. cit.). The Fas ligand is strongly expressed on activated lymphocytes, in the testis (T. Suda et al., op. cit.) and the eye (T. Griffith et al., Science 270:1189, 1995), as well as on some cytotoxic T-lymphocyte (CTL) cell lines (Rouvier et al., J. Exp. Med. 177:195, 1993).
Transfectant cells expressing FasL, as well as purified FasL protein (T. Suda and S. Nagata, J. Exp. Med. 179:873, 1994), are cytotoxic for cells expressing Fas. Thus, FasL transmits a signal for apoptosis by binding to Fas. More precisely, by analogy with the homologous TNF--TNF receptor system, whose molecular structure has been determined by X-ray crystallography (D. Banner et al., Cell 73:431, 1993), FasL is believed to function as a trimer. Also by analogy with TNF, the FasL trimer presumably binds one to three Fas molecules at the interface of respective FasL units (as schematically illustrated in FIG. 1). Binding of two or more Fas molecules to a FasL trimer presumably causes dimerization of Fas, which transmits an apoptotic signal to the Fas-expressing cell.
Fas-FasL induced cytotoxicity is one of the two major mechanisms of CTL-mediated cytotoxicity (D. Kagi, Science 265:528, 1994). The Fas system is believed to play an important role in the clonal deletion of peripheral autoreactive T cells and in control of the immune response (S. Nagata and T. Suda, Immunol. Today 16:39, 1995; J. Dhein et al., Nature 373:438, 1995), as mice with inactivating mutations in Fas (lpr mice) or FasL (gid mice) develop generalized lymphoproliferation and autoimmunity.
In addition, it has recently been discovered that mouse testis tissue transplanted into allogeneic mice is not rejected, presumably because the FasL expressed on the Sertoli cells of the testis destroys activated Fas-expressing T cells that would otherwise attack the transplanted tissue (D. Bellgrau, Nature 377:630, 1995). Similarly, expression of FasL in the eye is sufficient to destroy infiltrating inflammatory cells and make the eye an "immune privileged" site with reduced susceptibility to immune response and inflammation (T. Griffith et al., Science 270:1189, 1995). Also, cotransplantation of allogeneic pancreatic islet cells with myoblasts expressing FasL in mice protected the islet cells from immune rejection (H. Lau et al., Science 273:109, 1996).
This ability of FasL to destroy activated T cells suggests that it has potential as an immunosuppressive drug. However, FasL is likely to be highly toxic when injected into animals and humans, because it will induce apoptosis of other cells expressing Fas in addition to T cells, for example liver cells. Indeed, an agonistic antibody to murine Fas rapidly kills mice after intraperitoneal administration by causing massive necrosis of the liver, presumably mediated through apoptosis of hepatocytes via Fas (J. Ogasawara, Nature 364:806, 1993). Thus, compounds incorporating FasL that have specific cytotoxicity to autoimmune T cells and low non-specific toxicity are required for treatment of autoimmune disease and transplant rejection. The present invention fulfills these and other needs.